1. Field of the Invention
The present invention relates to a phase modulation device, phase modulation device fabrication method, crystallization apparatus, and crystallization method by which annealing such as uniform crystallization can be performed.
2. Description of the Related Art
The thin-film semiconductor technique is an important technique for forming semiconductor devices such as a thin-film transistor (TFT), contact type sensor, and photoelectric conversion element on an insulating substrate. The thin-film transistor is a field-effect transistor having a MOS (MIS) structure, and is also applied to a flat panel display such as a liquid crystal display (e.g., “Surface Science”, Vol. 21, No. 5, pp. 278-287).
The liquid crystal display generally has the characteristics as flatness, lightweight, and low power consumption, and can easily display color images. The liquid crystal display having these characteristics is widely used as displays of personal computers and various portable information terminals. When the liquid crystal display is an active matrix type display, thin-film transistors are used as pixel switching elements.
An active layer (carrier moving layer) of this thin-film transistor is made of, e.g., a thin silicon semiconductor film. The thin silicon semiconductor film is classified into amorphous silicon (a-Si) and polycrystalline silicon (non-single-crystal crystalline silicon) having fine crystal phases. The polycrystalline silicon is mainly polysilicon (poly-Si). Microcrystal silicon (μc-Si) is also known as the polycrystalline silicon. Examples of semiconductor thin-film materials other than silicon are SiGe, SiO, CdSe, Te, and CdS.
The carrier mobility in the active layer when a thin-film transistor is formed in polycrystalline silicon is about 10 times to 100 times as large as that when a thin-film transistor is formed in amorphous silicon. This carrier mobility characteristic is a superior characteristic as a semiconductor thin-film material for forming a TFT-structure switching element in a thin polycrystalline silicon film. Recently, a thin-film transistor using polysilicon as an active layer is noted for its high operating speed. This thin-film transistor having a high operating speed is noted as a switching element capable of forming various logic circuits such as a domino circuit and CMOS transmission gate. These logic circuits are necessary to form driving circuits of a liquid crystal display and electroluminescence display, a multiplexer, an EPROM, an EEPROM, a CCD, a RAM, and the like.
The conventional representative process of forming a thin semiconductor film made of polycrystalline silicon will be explained below. A substrate to be processed by this process has the following structure. An insulating substrate of glass or the like is prepared first. An undercoat layer (or buffer layer) such as a silicon oxide film (SiO2) is formed on this insulating substrate. In addition, an amorphous silicon film (a-Si) about 50 nm thick is formed as a thin semiconductor film on the undercoat layer. After that, dehydrogenation is performed to decrease the hydrogen concentration in the amorphous silicon film. Subsequently, a cap film such as a silicon oxide film (SiO2) is formed on the amorphous silicon film, thereby forming the substrate to be processed. Then, melt recrystallization of the amorphous silicon film is performed by excimer laser crystallization or the like. More specifically, an excimer laser beam irradiates the amorphous silicon film to change the amorphous silicon film in this irradiated region into a crystalline silicon film.
Presently, the thin polycrystalline silicon semiconductor film thus fabricated is used as an active layer (channel region) of an n- or p-channel thin-film transistor. In this case, the field-effect mobility (the electron or hole mobility obtained by the field effect) of the thin-film transistor is about 100 to 150 cm2/V·sec for the n-channel, and 100 cm2/V·sec for the p-channel. The use of this thin-film transistor makes it possible to form driving circuits such as a signal line driving circuit and scan line driving circuit on a substrate on which pixel switching elements are formed. Accordingly, a driving-circuit-intergrated display device can be obtained. As a consequence, the manufacturing cost of the display device can be reduced.
The electrical characteristics of the thin-film transistor formed on the insulating substrate are not so excellent as to integrate signal processing circuits, such as a D/A converter which converts digital video data into an analog video signal and a gate array which processes the digital video data, on the substrate of the display device. In this case, the thin-film transistor is required to have current driving capability two times to five times as high as the present capability. This thin-film transistor is also required to have a field-effect mobility of about 300 cm2/V·sec or more. To improve the functions and added values of the display device, the electrical characteristics of the thin-film transistor must be further improved. For example, when a static memory formed by a thin-film transistor is to be added to each pixel in order to give the pixel a memory function, this thin-film transistor is required to have electrical characteristics equivalent to those obtained when a single-crystal semiconductor is used. Therefore, it is important to improve the characteristics of the thin semiconductor film.
As a method of improving the characteristics of the thin semiconductor film, it is possible to make the crystallinity of the thin semiconductor film approach that of a single crystal. In effect, if the thin semiconductor film can be entirely changed into a single crystal on the insulating substrate, it is possible to obtain characteristics similar to those of a device using a SOI substrate which is examined as the next-generation LSI. This attempt was made more than 10 years ago as a three-dimensional device research project. Unfortunately, no technique capable of entirely changing the thin semiconductor film into a single crystal has been established yet. However, it is presently still expected that semiconductor grains in the thin semiconductor film be a single crystal.
Conventionally, a technique which grows large single-crystal semiconductor grains during crystallization of a thin amorphous semiconductor film is proposed (e.g., “Surface Science”, Vol. 21, No. 5, pp. 278-287). “Surface Science”, Vol. 21, No. 5, pp. 278-287 was announced as results of the research extensively continued by Matsumura et al. This reference discloses a technique which irradiates a thin amorphous semiconductor film with an excimer laser whose intensity is spatially modulated by using a phase shifter which modulates the phase of incident light. This reference also discloses a phase modulation excimer laser crystallization method which changes that region of the thin amorphous semiconductor film, which is irradiated with the laser into a thin polysilicon film by melt recrystallization. An ordinary laser crystallization method uses a laser beam having excimer laser intensity which is averaged (homogenized) on the plane of a thin silicon film by using an optical system (homogenizing optical system) called a beam homogenizer (e.g., “Flat Panel Display 96”, pp. 174-176). By contrast, the excimer laser intensity of the laser beam in the phase modulation excimer laser crystallization method disclosed in “Surface Science”, Vol. 21, No. 5, pp. 278-287 is varied on the plane of a thin silicon film by using the phase shifter, thereby producing a temperature gradient corresponding to this optical intensity distribution in the thin silicon film. This temperature gradient promotes the growth of single-crystal silicon grains from a low-temperature portion to a high-temperature portion in a lateral direction parallel to the plane of the thin silicon film. Consequently, this phase modulation excimer laser crystallization method can grow large-size, single-crystal silicon grains in the crystallized region, compared to the laser crystallization method disclosed in “Flat Panel Display 96”, pp. 174-176. This phase modulation excimer laser crystallization method can grow, in an amorphous silicon film, single-crystal silicon grains having a grain size of about a few μm by which one or a plurality of active elements such as thin-film transistors can be fabricated (accommodated). Accordingly, a thin-film transistor having electrical characteristics meeting the above-mentioned requirements can be obtained by forming the transistor in the single-crystal silicon grains thus grown.
The phase modulation excimer laser crystallization method disclosed in “Surface Science”, Vol. 21, No. 5, pp. 278-287 is an effective technique capable of forming large-size, single-crystal silicon grains in predetermined positions. The present inventors are extensively making research and development for applying this technique to industrial uses.
In the in-plane optical intensity distribution of a laser beam emitted from a laser source, the optical intensity is a maximum near the optical axis and decreases toward the periphery. Therefore, a crystallization apparatus generally has an optical system (homogenizing optical system) which homogenizes the optical intensity distribution of a laser beam in a two-dimensional plane.
Unfortunately, even when the excimer laser intensity is averaged by using the homogenizing optical system, the light irradiation intensity still decreases in the peripheral portion of the irradiation region. If crystallization is performed using this beam irradiation region, the irradiation intensity difference produces a difference between the sizes of crystal grains in the central portion and peripheral portion.
Furthermore, in the peripheral irradiation region in which the light irradiation intensity decreases, the silicon film does not reach the melting temperature and an annular non-crystallized region remains in a region where the irradiation intensity is too low.
When irradiation is performed a plurality of number of times in an irradiation region like this, even if low-light-irradiation-intensity portions between the adjacent irradiation regions are overlapped, no crystallization is well performed in the overlapped irradiation region. Accordingly, if a channel region of a thin-film transistor is formed in this region, the characteristics of the thin-film transistor worsen. When the substrate is to be entirely crystallized, therefore, it is required to densely irradiate the adjacent irradiation regions formed by repetitive irradiation.